1. Field of the Invention
The present invention generally relates to a transformer, and more particularly relates to a transformer that is used in a switching power supply device or a DC-DC converter device.
2. Description of the Related Art
A transformer is a main electronic component used in a switching power supply device or a DC-DC converter device. The transformer is also called a voltage converter or an electric transformer. In the transformer, a magnetic field is created by a primary coil, and the magnetic field is transferred to a secondary coil coupled with the primary coil by a mutual inductance, thereby inducing a current in the secondary coil. This allows an input voltage to be stepped up or stepped down.
FIG. 6 is a perspective view illustrating a typical EE type transformer 100. A first ferrite core 101 and a second ferrite core 102 shaped like the letter E in horizontal section each have a magnetic center leg at the center. A coil bobbin 103 has a primary coil and a secondary coil wound thereon. A shaft of the coil bobbin 103 is hollow, and the magnetic center legs are inserted through this hollow shaft. Tape 107 is wound around an outer perimeter of the first ferrite core 101 and the second ferrite core 102 in a horizontal direction. The horizontal direction is a direction parallel to a horizontal plane that contains an X axis and a Y axis illustrated in FIG. 6.
FIG. 7 is a horizontal sectional view of the typical EE type transformer. A gap 115 is provided between a magnetic center leg 118a of the first ferrite core 101 and a magnetic center leg 118b of the second ferrite core 102. A primary coil 104 and a secondary coil 105 are wound on the coil bobbin 103 so as to sandwich an interlayer sheet 106.
A typical assembly procedure of the transformer 100 is described below. First, the primary coil 104, a first interlayer sheet 106, the secondary coil 105, and a second interlayer sheet 106 are wound on the coil bobbin 103 in sequence. Next, a terminal process is performed. The coil bobbin 103 is inserted through the first ferrite core 101, and also inserted through the second ferrite core 102 from the opposite side. Lastly, to fix the first ferrite core 101 and the second ferrite core 102, the tape 107 is wound around the outer perimeter of these cores in the horizontal direction. After this, varnish impregnation is carried out. An unsaturated polyester resin, a modified polyester resin, an alkyd resin, or the like is used as a varnish. The transformer 100 is dipped (immersed) into a bath containing such a varnish for a specified time period, with the terminal facing upward. To solidify the varnish, the transformer 100 is maintained at a high temperature for several hours. By performing such a varnish impregnation process, the varnish penetrates and solidifies between the cores, between the coil bobbin and the cores, and between the coils and the interlayer sheets, thereby integrating these parts. In the transformer 100 made in this manner, the cores are resistant to breaking even when a heat cycle is repeated. Since the cores are entirely surrounded by the varnish, growl noise of the transformer 100 can be reduced.
Note that the growl noise of the transformer 100 can be reduced by adhering, with an adhesive, the facing magnetic center leg 118a of the first ferrite core 101 and the facing magnetic center leg 118b of the second ferrite core 102 to each other, a facing first magnetic outer leg 116a of the first ferrite core 101 and a facing first magnetic outer leg 116b of the second ferrite core 102 to each other, and a facing second magnetic outer leg 117a of the first ferrite core 101 and a facing second magnetic outer leg 117b of the second ferrite core 102 to each other. In particular, Japanese Patent Application Laid-Open No. H10-270261 proposes that abutting surfaces of the cores are adhered to each other. Japanese Patent Application Laid-Open No. 2005-057016 proposes that varnish impregnation is performed after the abutting surfaces of the cores are adhered to each other. Japanese Patent Application Laid-Open No. 2001-135529 proposes that a spacer is sandwiched between the abutting surfaces of the cores and also an elastic sheet is sandwiched between an upper surface of the bobbin and an inner surface of an upper core facing the upper surface of the bobbin and between a lower surface of the bobbin and an inner surface of a lower core facing the lower surface of the bobbin.
However, the magnitude of growl noise generated varies among transformers that have undergone impregnation. FIG. 8 is a diagram illustrating an acoustic spectrum of growl noise in a transformer having small growl noise. FIG. 9 is a diagram illustrating an acoustic spectrum of growl noise in a transformer having large growl noise. A horizontal axis represents a frequency, and a vertical axis represents a growl noise magnitude. In the case where the impregnant has reached the magnetic center legs 118a and 118b, the facing magnetic center legs 118a and 118b are adhered firmly to each other. Growl noise is small in such a transformer. In a transformer in which the impregnant has not reached the magnetic center legs 118a and 118b, on the other hand, growl noise is large.
FIG. 10 is a view for explaining a growl noise generation mechanism in a transformer in which the first magnetic outer leg 116a of the first ferrite core 101 and the first magnetic outer leg 116b of the second ferrite core 102 are not adhered to each other and the second magnetic outer leg 117a of the first ferrite core 101 and the second magnetic outer leg 117b of the second ferrite core 102 are not adhered to each other. The first magnetic outer legs 116a and 116b rub against each other, so that large growl noise is generated. Likewise, the second magnetic outer legs 117a and 117b rub against each other, so that large growl noise is generated. Meanwhile, there is a gap between the magnetic center leg 118a of the first ferrite core 101 and the magnetic center leg 118b of the second ferrite core 102, and therefore no rubbing occurs therebetween.
Accordingly, by adhering the facing first magnetic outer legs 116a and 116b to each other and also adhering the facing second magnetic outer legs 117a and 117b to each other, the rubbing can be suppressed, and as a result the growl noise can be reduced. FIGS. 11 and 12 are views illustrating natural vibrations of the magnetic outer legs. When the facing two magnetic outer legs are adhered to each other, growl noise is reduced, and natural vibrations 119a of the first magnetic outer legs 116a and 116b and natural vibrations 119b of the second magnetic outer legs 117a and 117b remain.
As described above, the following features are necessary in order to reduce growl noise.
Feature (1): vibrations are suppressed by adhering the facing magnetic center legs to each other.
Feature (2): noise caused by the rubbing between the magnetic outer legs is suppressed by integrating the magnetic outer legs with each other by means of adhesion or the like.
Feature (3): the natural vibrations of the magnetic outer legs are suppressed.
Moreover, the following transformer quality needs to be achieved.
Feature (4): there is a low possibility of core breaking caused by a difference in heat expansion coefficient between the coil bobbin and the ferrite cores.
However, according to the adhesion technique described in Japanese Patent Application Laid-Open No. H10-270261, the features (3) and (4) remain to be solved. According to the adhesion and impregnation technique described in Japanese Patent Application Laid-Open No. 2005-057016, the feature (3) remains to be solved. According to the elastic sheet technique described in Japanese Patent Application Laid-Open No. 2001-135529, the features (1), (2), and (3) remain to be solved because vibrations between the cores cannot be suppressed.
Various electrical devices nowadays are desired to be energy-saving with low power consumption. For example, to make an IC and the like in a power supply device energy-saving, more and more devices reduce the number of times the power supply device is switched during light load operation for improving efficiency. This can lead to a situation where a driving frequency of a transformer included in the power supply device becomes an audible frequency. Besides, while the electronic device is in light-load operation, its operation sound is small. This makes the growl noise of the transformer even more noticeable. For these reasons, there is a need to reduce the growl noise of the transformer caused by the natural vibrations of the magnetic outer legs.